Energy from sources that are renewable and ecologically safe, as opposed to sources that are non-renewable with toxic by-products, such as coal, oil, or gas (fossil fuels), and uranium (for nuclear power). The most important alternative energy source is flowing water, harnessed as
hydroelectric power. Other sources include the oceans' tides and waves (see
wave power),
wind power (harnessed by windmills and wind turbines), the Sun (
solar energy), and the heat trapped in the Earth's crust (
geothermal energy) (see also
cold fusion).
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